The present disclosure relates generally to well logging tools used in subterranean formations using acoustic waves and, more particularly, the present disclosure relates to methods and apparatus used to acoustically isolate the transmitter from the receiver.
Acoustic logging tools that are commonly used to characterize subterranean formations surrounding well bores. In general, acoustic logging tools operate by broadcasting an acoustic signal into a formation from one or more transmitters located at one position on the tool and receiving the signal with one or more receivers located at a second location on the tool. Properties of the received acoustic signal, such as travel time, frequency, amplitude, and attenuation, are then used to characterize the surrounding formation.
The transmitters generate a compressional waveform that travels through the wellbore fluids and into the surrounding formation. The acoustic wavefields propagate through the formation in a variety of modes, the most important being compressional waves, or “P-waves,” and transverse shear waves, or “S-waves.” P-waves are characterized by particle motion in the direction of wave travel while S-waves are characterized by particle motion perpendicular to the direction of wave travel. The energy mode characterized by particle motion perpendicular to the direction of wave travel is also called flexure mode. The various modes of propagation are distinguishable by their relative velocities. The velocities of both P-waves and S-waves depend on the elastic constants and the density of the medium through which the waves travel. There is a need in the art to attenuate the flexure mode in acoustic logging tools.
Ideally, the only acoustic signals received by the tool's receivers would be those signals that are transmitted by the tool that have traveled through the formation. However, if not properly isolated, the receiver will also detect other signals, sometimes referred to as “tool noise” or “road noise.” This undesired noise can interfere with the ability of the tool to render an accurate representation of the acoustic response of the formation. This noise is typically energy—more specifically, vibrations—traveling within or on the surface or body of the logging tool. The noise may be a high or low frequency noise, such as that created by the transmitters or by contact of the logging tool with the wellbore.
Acoustic isolators for downhole applications must be sufficiently flexible to attenuate acoustic waves traveling at or near the surface of the tool. Acoustic isolators must also be strong enough to survive running and retrieval operations, which may be by wireline or tubing conveyed means, for example. During these operations it is often required to push or pull heavy loads via the tool. There is a need in the art for improved isolators having advantageous frequency response characteristics, while having high tension and compression limits.
While embodiments of this disclosure have been depicted and described and are defined by reference to exemplary embodiments of the disclosure, such references do not imply a limitation on the disclosure, and no such limitation is to be inferred. The subject matter disclosed is capable of considerable modification, alteration, and equivalents in form and function, as will occur to those skilled in the pertinent art and having the benefit of this disclosure. The depicted and described embodiments of this disclosure are examples only, and not exhaustive of the scope of the disclosure.